The 300-word free tier is the usual stopping point — enough to see the interface, not enough to judge output at the length where these tools actually differ.
The other reason is the shape of the promise. An unlimited plan cannot be checked before you buy, and some people would rather hold a supplier to a published number than to a word.
A third is verification. Their site blocks automated reading, so the figures circulating about Phrasly — including ours — come from indexed pages rather than a direct look. That is a reason for care rather than suspicion, but it does make the plan harder to check than most.
None of that makes Phrasly a bad tool. It makes it a tool with a shape, and the question is whether that shape matches your work. Our full HumanFlow vs Phrasly comparison sets its numbers against ours in detail, including where Phrasly wins.